Twerk or the plural term "Twerking", means to shake the ass around in a fast, sexual act of arousal. Usually performed by hot females, usually having a large butt. Can be performed by men, but it's usually not meant to be sexual in any way.

Miley Cyrus, can't twerk for shit, she has no ass.
Did you see Jennifer twerk yesterday at the mall? She had such a nice ass!
Billy's mom can't twerk because she is just too old.

The act of standing with feet shoulder width apart while bending forwards at the waist, sticking out then shaking and jiggling the fat on your backside and upper thigh area. Does not work well on people without much body fat.